Various forms of phase modulation data transmission systems are presently known. In such systems, the phase of a carrier signal is varied in accordance with the coding of the data to be sent. The receiver unit which receives the transmitted signal, however, has no absolute sense of phase. It is, therefore, necessary either to use the signal itself in some way to generate information about the phases at the transmitting end of the system, or else to operate by examining the changes in phase that occur. The first approach needs a fixed reference signal at the receiver for identifying the reference phase used by the transmitter. A number of ingenious methods have been proposed for recovering the reference phase from the transmitted signal.
The second approach, which is referred to as a differential detection method, does not attempt to generate a fixed phase reference at the receiver. Instead, the data is coded by means of changes in the phase. The detector or demodulator at the receiver then merely looks for changes in phase from one interval to the next and does not need a phase reference signal. In this case, there is no need to have the coding start at any specific phase. If the phase of the carrier signal slips or drifts because of interference, the system will recover by itself. Thus, the differential detection method has definite advantages in terms of reduced equipment complexity.
A general description of phase modulation data transmission systems is given in the text book "Telecommunications And The Computer" (Second Edition), by James Martin, published by Prentice-Hall, Inc., 1976, at pages 224 -228 thereof.
Two basic methods have been heretofore proposed for demodulating a differential phase modulated carrier signal. One is a so-called coherent method (the same as is used for absolute phase encoded signals) and the other is a non-coherent method. In the coherent method, a carrier recovery circuit is used to reconstruct in-phase and quadrature-phase reference signals which are multiplied against the received signal and a phase shifted version of the received signal, with the results being linearly combined to produce a pair of demodulated signals representing the two modulation components of the received signal. The non-coherent demodulation method, on the other hand, does not use a carrier recovery circuit. Instead, a delayed version of the received signal is multiplied against the received signal to produce the demodulated signal.
The non-coherent demodulation method is usually simpler to implement since it does not require the carrier recovery circuit. In addition, the input filtering is less complex because a phase splitting filter is not required to generate the complex form of the received signal. The non-coherent method does, however, typically require a post detection filter in order to eliminate the double frequency terms generated by the multiplication process. Thus, each method has its advantages and disadvantages.
In digital implementations, wherein the demodulation functions are performed using digital number values obtained from a periodic sampling of the received signal, the coherent method appears to be the more attractive of the two because many of the calculations can be done at the symbol or baud rate, as opposed to the sampling rate. If a post detection filter is required, as is the case in the non-coherent method, the entire receiver must operate at the sampling rate in order to accurately filter out the double frequency terms generated by the demodulation process.
A typical coherent demodulator mechanism is described in a technical journal article entitled "Microprocessor Implementation Of High-Speed Data Modems", by P. J. Van Gerwen et al, appearing in the IEEE Transactions on Communications, Volume COM-25, Number 2, Feb. 1977, at pages 238 -250. Typical non-coherent demodulator mechanisms are described in the text book, "Digital and Analog Communication Systems", by K. S. Shanmugam, published by John Wiley & Sons, 1979.